Drop deprecation of `padding` parameter

[FabioLuporini]

configuration['autopadding'] = True makes devito applying padding to the innermost dimension such that the size is a multiple of the SIMD vector length

There are other reasons one may want to pad, e.g. to minimize cache conflicts. And clearly not all of them can be supported in Devito.

So we should drop the deprecation of padding: https://github.com/devitocodes/devito/blob/master/devito/types/dense.py#L929